Asbestos-reinforced cements have been widely used in a satisfactory manner in the building materials field for a number of decades. The building industry uses on a large scale a number of different processes for producing shaped articles such as pipes, corrugated boards and roofing slates, examples of the processes being the well known Magnani machine (cf Herbert Hiendl, "Asbestzementmaschinen", p. 42, 1964) and the equally well known Hatschek machine (see below). A preferred process, namely the winding process, an example of which is the Hatschek device, has been known for many decades (Austrian Pat. No. 5,970).
These known processes for producing articles such as asbestos cement pipes and boards are based on the use of specific dilute asbestos cement suspension in cylinder paper machines. The suspension is applied to a felt in fleece form via a drawing tank with the aid of a cylindrical-shaped screen and is wound to the desired thickness on size rollers or tubular cores.
For the production of corrugated boards, the asbestos cement fleece can be cut from the size roller on reaching the desired thickness and placed between lubricated corrugated metal sheets for setting purposes.
Over the last few years, it has been found that the asbestos which has been satisfactorily used in the known asbestos cement suspensions is not available in unlimited quantities and must be included among those natural materials whose supplies will probably be exhausted relatively soon. In addition, the deposits of workable asbestos are also only found in a few countries, which can in turn lead to undesired dependence problems.
Therefore, attempts have been made to find processes enabling the machines widely used in the asbestos cement industry to produce fiber-reinforced cement products without asbestos. Examples of such processes are described in published German patent applications 2,819,794, 2,816,457, 2,854,967 and 2,854,506, and in U.S. Pat. NO. 4,101,335. In all of these processes, operating on a basis of aqueous fiber-cement suspensions, an attempt is made to simulate the special characteristics of asbestos in the system by means of a mixture of different fiber types. The two most important characteristics of asbestos, upon which all drainage processes are based are: excellent filtration action, that is, the retaining capacity for cement; and the high strength of the asbestos fibers for providing a reinforcing action in the end product. Thus, it is a characteristic of known processes for producing asbestos-free fiber cement products on conventional machines to operate with mixtures of at least two different fiber types, one fiber type primarily for providing the filtering action and the second fiber type to provide reinforcement for the end product. The combination of the two fibers is an attempt to produce an end product equivalent to that using asbestos fibers.
Numerous different synthetic and natural fibers in cut lengths of 5 to 25 mm have been proposed as reinforcing fibers. Thus, fibers made of materials such as cotton, silk, wool, polyamide, polyester and polypropylene have been used, together with inorganic fibers such as those of glass, steel and carbon to provide the reinforcement. The following fibers, having primarily a filtering function, have previously been used: all types of cellulose, which may be in the form of pulp, mechanical wood pulp, waste paper, sawdust and waste from garbage disposal plants. In addition, "fibrides" based on polypropylene have been used, as well as inorganic filter fibers such as kaolin, rock or slag wool.
All hitherto known processes functioning with the aid of fiber systems which, in addition to the reinforcing fibers, also contain filtration fibers for the cement retaining capacity, have disadvantages. The relatively high proportion of filter fibers of 10 to 20% by volume required to ensure the necessary cement retaining capacity leads to undesired side-effects in the hydraulically setting or hardening matrix. The porosity of the products is increased and the contact between the reinforcing fibers and the binder is reduced, which leads to a reduction of product strength. If, in addition, cellulose-based filter fibers are used, the high swellability of such filtration systems mainly lead to a very considerable difference between the wet and dry strengths of the fiber cement products.
It would therefore be desirable to carry out production on the drainage systems used in the asbestos cement industry without filter fibers, that is, with reinforcing fibers only.